1. Field of the Invention
This invention relates to aircraft flight controls and, more particularly, to actuating an apparatus which overrides a part of the flight control system of an aircraft.
2. Descripton of the Prior Art
Modern aircraft are equipped with electrical flight control (also known as "fly-by-wire") systems which do not translate high aerodynamic hinge moments or loads directly into a proportionally high force acting on the control stick. As a result, a pilot is physically able to command control surface deflections which will generate normal acceleration capable of causing structural failure of the aircraft or black-out of the pilot. To guard against this danger, the flight control systems incorporated into the current generation of aircraft include a g-limiter which prevents the pilot from commanding normal g's in excess of a predetermined level of safety. However, situations can arise where the pilot may choose to exceed the design limit load of his aircraft by commanding additional normal g's in order to avoid crashing into the ground or colliding with another aircraft, or to evade a hostile aircraft or missile.
In order to provide the pilot with such an option, modern flight control systems include an override for the g-limiter that can be actuated by the pilot. The ultimate strength of the aircraft at which structural failure will occur is typically 150% of the design limit load. Thus additional g's in excess of the design limit load, but still short of the ultimate strength, are available for such emergencies.
One apparatus currently used to actuate a g-limiter override is comprised of a paddle switch located on the control stick. The switch is electrically connected to g-limiter override circuitry contained in the flight control computer. In order to actuate the g-limiter override, the fifth digit (pinky finger) of the gloved hand that is gripping the control stick must be extended and hooked around a rotatable paddle, and the paddle must then be pulled towards the control stick. This maneuver requires unusual dexterity or the use of a second hand. In an emergency high-g situation, the condition typically extant when it might be necessary to override the g-limiter, the g's hinder the pilot from extending his fifth digit and from pulling the paddle, or from using a second hand. In contrast to the movement necessary to operate the foot brake in an automobile, the movement of the digit, and certainly the use of a second hand, are not reflexive actions, and thus pulling the paddle will require extra time even under optimal circumstances.
The use of hydraulics in an attempt to solve the problem is exemplified by U. S. Patent No. 2,849,198: "Hydraulic Force-Limiter for Aircraft Controls," issued on Aug. 26, 1958, to Otto J. Borngesser. When the aircraft is subjected to excessive g-forces, the disclosed mechanism increases the resistance against movement of the control stick, with the resistance increasing proportionally with the g-forces. The invention includes a g-force limiter which can be overridden by the application of an abnormally large force on the control stick. The abnormally large force necessary to overcome the resistance must be continuously applied to the control stick in order to keep the override engaged, and thus hinders lateral stick movement during operation of the override as well as tiring the pilot.
Another approach is disclosed by U. S. Patent No. 2,903,205: "Electric `G` Limiter for Aircraft", issued on Sept. 8, 1959 to Otto J. Borngesser and Alfred J. Monroe. This invention uses a magnetic clutch to generate a relatively high opposing or inhibiting force on the control stick when the movement of the control stick by the pilot would otherwise increase the structural load on the aircraft beyond the design limit load. When necessary, the pilot may overcome the g-limiter and continue movement of the control stick because the limiting torque output of the clutch is always less than the maximum force that can be applied by the operator. As in the case of the previously discussed hydraulic g-limiter, a resisting torque is continually applied by the magnetic clutch while the control system is operating in the override mode and thus similarly impedes lateral stick movement during operation of the override and tires the pilot.